CMOS Batteries

All 16-bit and higher systems have a special type of chip in them that combines a real-time clock (RTC) with at least 64 bytes (including the clock data) of nonvolatile RAM (NVRAM) memory. This chip is officially called the RTC/NVRAM chip, but it is often referred to as the CMOS or CMOS RAM chip because the type of chip used is produced using a CMOS (complementary metal-oxide semiconductor) process.

CMOS design chips are known for very low power consumption. This special RTC/NVRAM chip is designed to run off a battery for several years. The original chip of this type used in the IBM AT was the Motorola MC146818 chip. Although the chips used today have different manufacturers and part numbers, they all are designed to be compatible with this original Motorola part.

Most modern motherboards have the RTC/NVRAM built in to the motherboard chipset South Bridge or I/O Controller Hub (ICH) component. The function of the real-time clock should be obvious: The clock enables software to read the date and time and preserves the date and time data even when the system is powered off or unplugged.

The NVRAM portion of the chip has another function. It is designed to store basic system configuration, including the amount of memory installed, types of floppy and hard disk drives, and other information. Some of the more modern motherboards use extended NVRAM chips with as much as 4KB or more of space to hold this configuration information.

This is especially true for plug-and-play systems, which store the motherboard configuration as well as the configuration of adapter cards. The system can then read this information every time you power it on. These chips typically are powered by some type of battery while the system is off.

This battery preserves the information in the NVRAM and powers the clock. Most systems use a lithium-type battery because they have a very long life, especially at the low power draw from the typical RTC/NVRAM chip. Some systems have a chip that has the battery embedded within it.

These are made by several companies—including Dallas Semiconductor and Benchmarq. These chips are notable for their long lives. Under normal conditions, the integral battery lasts for 10 years—which is, of course, longer than the useful life of the system. If your system uses one of the Dallas or Benchmarq modules, the battery and chip must be replaced as a unit because they are integrated.

Most of the time, these chip/battery combinations are installed in a socket on the motherboard just in case a problem requires an early replacement. You can get new modules direct from the manufacturers for $18 or less, which is often less than the cost of the older separate battery alone.

Some systems do not use a battery at all. Hewlett-Packard, for example, includes a special capacitor in some of its systems that is automatically recharged anytime the system is plugged in. Note that the system does not have to be running for the capacitor to charge; it only has to be plugged in.

If the system is unplugged, the capacitor powers the RTC/NVRAM chip for up to a week or more. If the system remains unplugged for a duration longer than that, the NVRAM information is lost. In that case, these systems can reload the NVRAM from a backup kept in a special flash ROM chip contained on the motherboard.

The only pieces of information that will actually be missing when you repower the system are the date and time, which will have to be reentered. By using the capacitor combined with an NVRAM backup in flash ROM, these systems have a very reliable solution that lasts indefinitely.

Many systems use a separate battery, which can be either directly soldered into the motherboard (mostly older obsolete systems) or plugged in via a battery connector. For those older systems with the battery soldered in, a spare battery connector exists on the motherboard where you can insert a conventional plug-in battery, should the original ever fail.

Motherboard NVRAM (CMOS RAM) batteries come in many forms. The best are of a lithium design because they last 2–5 years or more. I have seen systems with conventional alkaline batteries mounted in a holder; these are much less desirable because they fail more frequently and do not last as long.

Also, they are prone to leak, and if a battery leaks on the motherboard, the motherboard can be severely damaged. By far, the most commonly used battery for motherboards today is the type 2032 lithium coin cell battery, which is about the size of a quarter and is readily available at electronics supply stores, camera shops, and even drugstores.

Besides the various battery types, the chip can require any one of several voltages. The batteries in PCs are usually 3.0V, but some systems have used 3.6V, 4.5V, or 6V types as well. If you are replacing the battery, be sure your replacement is the same voltage as the one you removed from the system.

Some motherboards can use batteries of several voltages, and you use a jumper or switch to select the various settings. If you suspect your motherboard has this capability, consult the documentation for instructions on changing the settings. Of course, the easiest thing to do is to replace the existing battery with another of the same type.

Symptoms that indicate that the battery is about to fail include having to reset the clock on your PC every time you shut down the system (especially after moving it) and problems during the system's POST, such as drive-detection difficulties. If you experience problems such as these, you should make note of your system's CMOS settings and replace the battery as soon as possible.

When you replace a battery, in most cases the existing data stored in the NVRAM is lost. Sometimes, however, the data remains intact for several minutes (I have observed NVRAM retain information with no power for an hour or more), so if you make the battery swap quickly, the information in the NVRAM might be retained.

Just to be sure, I recommend that you record all the system configuration settings stored in the NVRAM by your system Setup program. In most cases, you should run the BIOS Setup program and copy or print out all the screens showing the various settings.

Some Setup programs offer the capability to save the NVRAM data to a file for later restoration if necessary. After replacing a battery, power up the system and use the Setup program to check the date and time setting and any other data that was stored in the NVRAM.